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Sybex CCNA 640-802 Chapter 6: IP Routing. Chapter 6 Objectives. Understanding IP routing Static routing Default routing Dynamic routing RIP RIPv2 IGRP Verifying routing

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Sybex CCNA 640-802

Chapter 6: IP Routing

chapter 6 objectives
Chapter 6 Objectives
  • Understanding IP routing
  • Static routing
  • Default routing
  • Dynamic routing
    • RIP
    • RIPv2
    • IGRP
    • Verifying routing
    • [Oddly, the exam topics covered in this chapter (6) are listed at the beginning of the chapter. Some of the topics listed are not really covered in this chapter at all. For example, OSPF and EIGRP are covered in chapter 7, not chapter 6. ]


what is routing
In order to “route”, a router needs to know:

Remote Networks

Neighbor Routers

All Possible routes to remote network

The absolute best routeto all remote networks

Maintain and verify the routing information

Remember: a router does not deal with hosts!

A router only deals with networks, and the best path to them

An IP address allows packets to move from network to network

Hardware (Mac) addresses move the packets to specific hosts

What is Routing?





basic path selection
Basic Path Selection

On what interface will the router send out a packet if it has destination address of

simple ip routing
Simple IP Routing









Host A

Host B

routing pdu example host a web browses to the http server
Routing/PDU Example:Host A Web browses to the HTTP Server….

1. The destination address of a frame will be the: Host A address

2. The destination IP address of a packet will be the IP address of the: Destination Router

3. The destination port number in a segment header will have a value of 80 (the port number used by HTTP)

idea of routing 5 guest slides
Idea of routing (5 guest slides)
  • Routers forward datagrams between connected networks
  • They need to know via which interfaceto send a datagram
  • Routing decisions are based on the information stored in the routing table
routing table
Routing table
  • Tells where to send datagram for a particular network

NetworkNext-HopPortMetric Eth0 1 Eth1 14 Fddi0 8 S0 5

  • “Next-Hop” routers must be directly reachable
routing table cont
Routing table (cont.)
  • Default Route - a special entry in the routing table:
    • “Pass all datagrams for unknown networks to this router”
    • Represented by the entry for network
  • Routing uses networkpart of the address!
step by step ip routing process book pp 331 36
Step-by-Step: IP Routing Process (book, pp 331-36)
  • The IP routing process is fairly simple and doesn’t change, regardless of the size of your network.
  • For an example, we’ll use Figure 6.2 to describe step-by-step what happens when Host_A wants to communicate with Host_B on a different network
step 1
Step 1
  • Internet Control Message Protocol (ICMP) creates an “echo request” payload (which is just the alphabet in the data field).
    • The echo request is the first part/half of what is commonly called a “Ping”; the second part is the echo reply, from the device being “pinged”.
  • [So, A is going to “ping” B]
step 2
Step 2
  • ICMP hands that payload to Internet Protocol (IP), which then creates a packet.
  • At a minimum, this packet contains an IP source address, an IP destination address, and a Protocol field with 01h.
    • (Remember that Cisco likes to use 0x in front of hex characters, so this could look like 0x01.)
  • All of that tells the receiving host to whom it should hand the payload when the destination is reached—in this example, ICMP.
step 3
Step 3
  • Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one.
step 4
Step 4
  • Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so the packet can be routed to the remote network.
  • The Registry in Windows is “parsed” to find the configured default gateway.
step 5
Step 5
  • The default gateway of host (Host_A) is configured to For this packet to be sent to the default gateway, the hardware address of the router’s interface Ethernet 0 (configured with the IP address of must be known.
  • Why? So the packet can be handed down to the Data Link layer, framed, and sent to the router’s interface that’s connected to the network.
  • Because hosts only communicate via hardware addresses on the local LAN, it’s important to recognize that for Host_A to communicate to Host_B, it has to send packets to the Media Access Control (MAC) address of the default gateway.
step 6
Step 6
  • Next, the Address Resolution Protocol (ARP) cache of the host is checked to see if the IP address of the default gateway has already been resolved to a hardware address. Two possibilities ensue:
  • 1. If it has, the packet is then free to be handed to the Data Link layer for framing. (The hardware destination address is also handed down with that packet.) To view the ARP cacheon your host, use the following command:
  • C:\>arp -a
  • Interface: --- 0x3
  • Internet Address Physical Address Type
  • 00-15-05-06-31-b0 dynamic
  • 2. If the hardware address isn’t already in the ARP cache of the host, an ARP broadcastis sent out onto the local network to search for the hardware address of The router responds to the request and provides the hardware address of Ethernet 0, and the host caches this address.
Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used (in this example, Ethernet). ALAN driver provides communication control between the NOS and NIC (network interface card).
  • A frame is then generated, encapsulating the packet with control info.
  • Within that frame are the hardware destinationand source addresses plus, in this case, an Ether-Type field that describes the Network layer protocol that handed the packet to the Data Link layer—in this instance, IP.
  • At the end of the frame is that Frame Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC).
  • The frame would look something like what is detailed in Figure 6.3. It contains Host_A’s hardware (MAC) address and the destination hardware address of the default gateway. It does not include the remote host’s MAC address—remember that!

FIGURE 6 . 3

Frame used from Host_A to the Lab_A router when Host_B is pinged

step 7
Step 7

FIGURE 6 . 3

Frame used from Host_A to the Lab_A router when Host_B is pinged

step 10
Step 10
  • The packet is pulled from the frame, and what is left of the frame is discarded.
  • The packet is handed to the protocol listed in the Ether-Type field — i.e., it’s given to IP.
    • [So now the packet is at the router, having entered at interface E0, the default gateway for the network.
    • Next, the router will try to send the packet to its destination in the network.
    • To do so, it will have to find this network in its routing tables.]
step 11
Step 11
  • IP receives the packet and checks the IP destination address.
  • Since the packet’s destination address doesn’t match any of the addresses configured on the receiving router itself, the router will look up the destination IP network address in its routing table.
step 12
Step 12
  • The routing table must have an entry for the network or the packet will be discarded immediately and an ICMP message will be sent back to the originating device with a “destination network unreachable” message.
    • [Note that 172.16.x.x is a Class B network. .10 and .20 would ordinarily be part of the same network and therefore couldn’t be set up on 2 networks. But this network is subnetted, i.e., the subnet mask is
step 13
Step 13
  • If the router does find an entry for the destination network in its table, the packet is switched to the exit interface—in this example, interface Ethernet 1.
  • The outputbelow (next slide) displays the Lab_A router’s routing table. The “C” means “directly connected.”
  • No routing protocols are needed in this network since all (both) networks are directly connected.
step 13 continued
Step 13 (continued)
  • Lab_A>sh ip route
  • Codes: C – connected , S – static , I - IGRP,R - RIP,M - mobile, – BGP, D - EIGRP,EX - EIGRP external,O - OSPF,IA - OSPF inter area, N1 - OSPF NSSA external type 1, N2-OSPF NSSA external type 2, E1 - OSPF external type 1, E2 - OSPF external type 2, E – EGP, i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS intearea * - candidate default, U - per-user static route, o – ODR P - periodic downloaded static route
  • Gateway of last resort is not set
  • is subnetted, 2 subnets
  • C is directly connected, Ethernet0
  • C is directly connected, Ethernet1
step 14
Step 14
  • The router packet-switches the packet to the Ethernet 1 buffer.
    • [OK, ready to go out to Host_B, but first …]
step 15
Step 15
  • The Ethernet 1 buffer needs to know the hardware address of the destination host and first checks the ARP cache.
    • If the hardwareaddress of Host_B has already been resolved and is in the router’s ARP cache, then the packet and the hardware address are handed down to the Data Link layer to be framed.
    • Let’s take a look at the ARP cache on the Lab_A router by using the “show ip arp” command:
  • Lab_A#sh ip arp
  • Protocol Address Age(min) Hardware Addr Type Interface
  • Internet - 00d0.58ad.05f4 ARPA Ethernet0
  • Internet 3 0030.9492.a5dd ARPA Ethernet0
  • Internet - 00d0.58ad.06aa ARPA Ethernet0
  • Internet 12 0030.9492.a4ac ARPA Ethernet0
    • The dash(-) means that this is the physical interface on the router.
step 15 continued
Step 15 (continued)
  • From the output in the previous slide, we can see that the router knowsthe (Host_A) and (Host_B)hardware addresses.
    • Cisco routers will keep an entry in the ARP table for 4 hours.
  • If the hardware address has not already been resolved, the router sends an ARP request out E1 looking for the hardware address of
  • Host_Bresponds with its hardware address, and the packet and destinationhardware address are both sent to the Data Link layer for framing.
step 16
Step 16
  • The Data Link layer creates a frame with the destination and sourcehardware address, Ether-Type field, and FCS field at the end.
    • [Still a small packet – just four fields]
  • The frame is handed to the Physical layer to be sent out on the physical medium one bit at a time.
    • [Now we see packets actually going to Host_B]
step 17
Step 17
  • Host_Breceives the frame and immediately runs a CRC. [finally!!]
  • If the result matcheswhat’s in the FCS field, the “hardware destination address” is then checked. If the host finds a match, the Ether-Type field is then checked to determine the protocol that the packet should be handed toat the Networklayer — IP in this example.
    • [IP is by far the most common Layer 3 protocol.]
    • [Moving up the OSI model. Data Link to Network]
step 18
Step 18
  • At the Network layer, IP receives the packet and checks the IP destination address.
  • Since there’s finally a match made, the Protocol field is checked to find out to whom the payload should be given.
step 19
Step 19
  • The payload is handed to ICMP, which understands that this is an echo request.
  • ICMP responds to this by immediately discarding the packet and generating a new payload as an echo reply.
step 20
Step 20
  • A packet is then created, including the
    • source and destinationaddresses,
    • Protocolfield, and
    • payload.
  • The destination device is now Host_A
step 21
Step 21
  • IP then checks to see whether the destination IP address is a device on the local LAN or on a remote network.
  • Since the destination device is on a remote network, the packet needs to be sent to the default gateway.
step 22
Step 22
  • The default gateway IP addressis found in the Registry of the Windows device, and the ARP cache is checked to see if the hardware address has already been resolved from an IP address.
    • You can search the Registry by going into the Registry Editor (start/Run/regedit), then searching for “DefaultGateway” (F3 – enter search parameters).
    • See “Default” / “DHCP Default Gateway” next slide
step 22 continued
Step 22 (continued)

Above is a view of my home computer’s Registry settings: HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\longkey\Parameters\Tcpip

step 23
Step 23
  • Once the hardware address of the default gateway is found, the packet and destinationhardware addresses are handed down to the Data Link layer for framing.
step 24
Step 24
  • The Data Link layer frames the packet of information and includes the following in the header:
    • The destination & source hardware addresses
    • The Ether-Type field [with 0x0800 (IP) in it]
    • The FCS field with the CRC result in tow
step 25
Step 25
  • The frame is now handed down to the Physical layer to be sent out over the network medium one bit at a time.
step 26
Step 26
  • The router’s Ethernet 1 interface receives the bits and builds a frame.
  • The CRC is run, and the FCS field is checked to make sure the answers match.
step 27
Step 27
  • Once the CRC is found to be okay, the hardware destination address is checked.
  • Since the router’s interface is a match, the packet is pulledfrom the frame and the Ether-Type field is checked to see to what protocol at the Network layer the packet should be delivered.
step 28
Step 28
  • The protocol is determined to be IP, so it gets the packet.
  • IP runs a CRC check on the IP header first and thenchecks the destination IP address.
    • IP does not run a completeCRC as the Data Link layer does—it onlychecks the header for errors.
step 29
Step 29
  • In this case, the router does know how to get to network — the exit interface is Ethernet 0 — so the packet is switched to interface Ethernet 0.
step 30
Step 30
  • The router checks the ARP cache to determine whether the hardware address for has already beenresolved.
step 31
Step 31
  • Since the hardware address to is already cached from the originating trip to Host_B, the hardware address and packet are handed to the Data Link layer.
step 32
Step 32
  • The Data Link layer builds a frame with the destination hardware address and source hardware address and then puts IP in the Ether-Typefield.
  • A CRC is run on the frame and the result is placed in the FCS field.
step 33
Step 33
  • The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
step 34
Step 34
  • The destination host receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out to whom to hand the packet.
step 35
Step 35
  • IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the protocol field for further direction.
  • IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply.
step 36
Step 36
  • ICMP acknowledges that it has received the reply by sending an exclamation point(!) to the user interface.
  • ICMP then attempts to send four more echo requests to the destination host.
  • The End
post script
Post Script
  • These steps are the basic routing process, no matter how large the network.
    • There would just be more hops in a big internetwork.
  • Point to recap:
    • Moving from router to router in a big internetwork, at each hop the hardware address changes; from one router’s Mac address to the next’s.
    • But from hop to hop, the IP address remains the same!
    • This reflects the fact that hardware addresses (Mac) are always local, while logical addresses (IP, for example), are always remote.
      • I.e., in a local LAN, you always use a Mac addrss, not IP.
This is a project that runs from pp 336 to 362.
  • Setup: 5 Routers and an wireless Access Point
  • Neither of our network simulators has these routers, so all we can do is read over the configurations.
  • Notes:
    • P.345: With an ISR router, no need to use the “clock rate” command; they automatically detect it.
    • P346: See the interface “serial 0/0/1”. The book explains the way interfaces are labeled in a couple of places:
      • Pg 184 and 195: “x/y/z Slot/Subslot/Port” (brief)
Notes: (continued)
    • Page 205: Better explanation here:
    • Some modular routers use three numbers instead of two.
    • The first 0 is the router itself, and then you choose the slot, and then the port. Here’s an example of a serial interface on a 2811:
      • Todd(config)#interface serial ?
      • <0-2> Serial interface number
      • Todd(config)#interface serial 0/0/?
      • <0-1> Serial interface number
      • Todd(config)#interface serial 0/0/0
      • Todd(config-if)#
Notes: (continued)
    • You should always view a running-config output first so you know what interfaces you have to deal with. Here’s a 2801 output:
        • Todd(config-if)#do show run
        • Building configuration...
        • [output cut]
        • !
        • interface FastEthernet0/0
        • no ip address
        • Shutdown
        • duplex auto
        • speed auto
        • !
        • interface FastEthernet0/1 [continued on next slide]
no ip address
  • shutdown
  • !
  • interface Serial0/2/0
  • no ip address
  • shutdown
  • clock rate 2000000
  • !
  • [output cut]
  • no ip address
  • shutdown
  • duplex auto
  • speed auto
  • !
  • interface Serial0/0/0
  • no ip address
  • shutdown
  • no fair-queue
  • !
  • interface Serial0/0/1
  • no ip address
  • shutdown
  • !
  • interface Serial0/1/0
  • [continued in next column]
At other times you may see a x/x/x config for modular units (like WICs) where you have a slot, a subslot, and a port. From
    • “The slot/subslot/port format only applies to WIC interfaces. Interfaces that are native to the network modules still use only the slot/port format. That is:
      • slot/port is used whenever the interfaces are native on the network module.
      • slot/subslot/port is used whenever the interfaces are on the WIC slot of a network module (NM).”
  • There are still more examples where the interface is a 3-part config.
Notes: (continued)
    • Pg 346-47: Just a command idiosyncrasy:
    • With ISR routers you can’t use “erase start”, you must enter “erase startup-config”
    • This is so even though no other command begins with “S”:
      • Eg: Router#erase s?
      • startup-config
      • So under the normal rules of the Cisco IOS, “erase s” should work exactly like “erase startup-config”, but it doesn’t.
    • This is probably just an oversight that will be corrected in the next IOS version. Just be aware that you will sometimes find anomalies like this.
Notes: (continued)
    • Pg 351 ff: Wireless interfaces: 2 things unique to them:
      • SSID #: “The Service Set Identifier that creates a wireless network that hosts can connect to.”
      • DHCP Pool for wireless clients: Actually just like DHCP with wired clients. More on this in Chapter 12.
    • Pg 352 ff: Author uses the SDM here – “Security Device Manager” to configure interface R3 in the example.
      • The book goes through a series of steps using the SDM’s wizard – through page 359.
configuring ip routing in our network
Configuring IP Routing in Our Network
  • Even after the previous pages/slides, we still we need to do some things to get our network up to speed.
  • 3 things to do:
    • Static Routing
    • Default Routing
    • Dynamic Routing
static routes
Static Routes

Stub Network







Routes must be unidirectional

static route configuration
Static Route Configuration

ip route remotenetwork



[distance] - all static routes have a distance of “1”; very trustworthy

[permanent]- to keep the route in the table no matter what;

even if the interface goes down.

Router(config)#ip route remote_network mask next_hop

This means: to get here (ip address and mask) go here next (address only)


You can optionally add a distance if you want to change the metric of the route; for example, you may want to prefer any dynamic route

static route example
Static Route Example

Stub Network






ip route


ip route s0

default routes
Default Routes

Stub Network



creates a wireless network that hosts can connect to.




To send packets with a remote destination network not in the routing table to the next-hoprouter, only used for stub networks.

ip route

ip classless

[Note: This configuration sends every packet out Router A’s 3.1 interface]

static route considerations
Static Route Considerations

When configuring static routes, consider the following:

By default, a static route will take precedence over a dynamic route because of its lower administrative distance.

Without additional configuration, a dynamic route to a network will be ignored if a static route is present in the routing table for the same network.

To reduce the number of static route entries, define a summarized or default static route

static route considerations1
Static Route Considerations
  • The benefit of using static routes is that they do not require the router to spend CPU cycles and memory space to determine the best route to a destination. The route has already been placed in the routing table manually.
  • This can work against the network, however, if a device in the static route’s path goes down. In this case, the packets may still attempt to use the path (especially if the “permanent” option is chosen), and in any event, no other route will be chosen, as in a dynamic routing network, because the static route has limited the choices.
routing protocols dynamic
Routing Protocols (Dynamic)
  • Routing protocols are used between routers to:
    • Determine the path of a packet through a network
    • Maintain routing tables
    • Two types:
      • Interior gateway protocols (IGPs)
      • exteriorgateway protocols (EGPs)
      • Examples:
        • EGP: Border Gateway Protocol (BGP)
  • [Note: This is only one way to distinguish between routing protocols; others include: distance vector v. link state, and we’ve already begun to distinguish static v. dynamic]

72 / 377

routing protocols
Routing Protocols



Autonomous System 1

Autonomous System 2

  • An autonomous system is a collection of networks under a “common administrative domain”, i.e., all routers sharing the same routing table are in the same AS.
  • IGPs operate within an autonomous system.
  • EGPs connect different autonomous systems.
classful routing overview
Classful Routing Overview
  • “Classful” routing protocols do not include the subnet mask with the route advertisement.
  • Within the same network, consistency of the subnet masks is assumed.
  • Summary routes are exchanged between foreign networks.
  • Examples of classful routing protocols:
    • RIP Version 1 (RIPv1)
    • IGRP
  • [The problem with classful routes is that they don’t
classless routing overview
Classless Routing Overview
  • Classless routing protocols include the subnet mask with the route advertisement.
    • Classless routing protocols support variable-length subnet masking (VLSM).
    • Summary routes can be manually controlled within the network.
    • Examples of classless routing protocols:
      • RIP Version 2 (RIPv2)
      • EIGRP
      • OSPF
      • IS-IS
classful versus classless routing protocols
Classful Versus Classless Routing Protocols
  • A classful routing protocol always considers the IP network class
    • Address summarization is automatic by major network number and discontiguous subnets are not visibleto each other
  • Classless protocols transmit prefix-length or subnet mask information with IP network addresses.
    • The IP address can be mapped so that discontinuous subnets and VLSM are supported
administrative distance
Administrative Distance

Default Administrative Distance

Directly Connected: 0

Static Route: 1

RIP: 120

IGRP: 100


OSPF: 110

Router B

Router A





Router C

Router D

The administrative distance (AD) is used to rate the trustworthiness of routing informationreceived on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.

If both advertised routes to the same network have the same AD, then routing protocol will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. If it’s a tie, load balancing is used.


distance vector
Distance Vector

Distance—How farVector—In which direction













All routers just broadcast their entire routing table out allactive interfaces on periodic time intervals

Distance vector algorithmsdo not allow a router to know the exact topology of an internetwork.

78 / 379

discovering routes converged routing tables
Discovering Routes: Converged Routing Tables

By “converged” we mean that each of the routers above has the same view of the internetwork, i.e., each router sees the same number of links from one router to any other router.

meaning of distance vector 1 2
Meaning of Distance Vector (1/2)
  • A router using a distance vector routing protocol does not have the knowledge of the entire pathto a destinationnetwork.
  • The router only knows
    • The direction or interface in which packets should be forwardedand
    • The distance or how far it is to the destination network
operation of distance vector 1 4
Operation of distance vector (1/4)
  • Some distance vector routing protocols call for the router to periodically broadcast the entire routing table to each of its neighbors.
  • This method is inefficient because the updates not only consume bandwidth but also consume router CPU resources to process the updates.
operation of distance vector 2 4
Operation of distance vector (2/4)
  • Periodic Updatesare sent at regular intervals (30 seconds for RIP and 90 seconds for IGRP).
    • Even if the topologyhas not changed in several days, periodic updates continue to be sent to all neighbors.
    • Neighbors are routers that (1)share a link and are configured to (2) use the same routing protocol.
    • The router is only aware of the network addresses of its own interfaces and the remote network addresses it can reach through its neighbors
operation of distance vector 3 4
Operation of distance vector (3/4)
  • Broadcast Updatesare sent to
    • Neighboring routers that are configured with the same routing protocol will process the updates.
    • All otherdevices will also process the update up to Layer 3 before discarding it.
    • Some distance vector routing protocols use multicast addresses instead of broadcast addresses.
operation of distance vector 4 4
Operation of distance vector (4/4)
  • Entire Routing Table Updatesare sent, periodically to all neighbors.
    • Neighbors receiving these updates must process the entire update to find pertinent information and discard the rest.
    • Some distance vector routing protocols like EIGRPdo not send periodic routing table updates.
routing algorithm
Routing Algorithm
  • The algorithm used for the routing protocols defines the following processes:
    • Mechanism for sending and receiving routing information.
    • Mechanism for calculating the best paths and installing routes in the routing table.
    • Mechanism for detecting and reacting to topology changes.
routing protocol characteristics 1 3
Routing protocol characteristics (1/3)
  • Time to Convergence- Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge.
    • The faster the convergence, the more preferable the protocol.
    • Routing loops can occur when inconsistentrouting tables are not updated due to slow convergence in a changing network.
routing protocol characteristics 2 3
Routing protocol characteristics (2/3)
  • Scalability- Scalability defines how large a network can become based on the routing protocol that is deployed.
    • The larger the network is, the more scalable the routing protocol needs to be.
  • Classless (Use of VLSM) or Classful - Classless routing protocols include the subnet mask in the updates.
    • This feature supports the use of Variable Length Subnet Masking (VLSM) and better route summarization.
    • Classful routing protocols do not include the subnet mask and cannot support VLSM.
routing protocol characteristics 3 3
Routing protocol characteristics (3/3)
  • Resource Usage- Resource usage includes the requirements of a routing protocol such as memoryspace, CPU utilization, and link bandwidth utilization
    • Higher resource requirements necessitate more powerful hardware to support the routing protocol operation in addition to the packet forwarding processes.
  • Implementation and Maintenance - Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.
routing loops 1 6
Routing Loops (1/6)
  • A routing loop is a condition in which a packet is continuously transmitted within a series of routerswithout ever reachingits intended destination network.
  • A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid pathto an unreachable destination exists.
routing loop 2 6
Routing Loop (2/6)
  • The loop may be a result of:
    • Incorrectly configured static routes
    • Incorrectly configured route redistribution(redistribution is a process of handing the routing information from one routing protocol to another routing protocol)
    • Inconsistent routing tables not being updated due to slow convergence in a changing network
    • Incorrectly configured or installed “discard routes”
routing loops ways to stop them
Routing Loops & Ways to Stop Them
  • Maximum hop count, AKA, Counting to Infinity: RIP permits a hop count of up to 15. At 16 hops, a route is considered to be an infinite distance away.
  • This is called counting to infinity, and it’s caused by gossip(broadcasts) and wrong information being communicated and propagated throughout the internetwork.
  • Without some form of intervention, the hop count increasesindefinitely each time a packetpasses through a router.

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count to infinity 1 5
Count to infinity (1/5)
  • Count to infinity is a condition that exists when inaccurate routing updates increase the metric value to "infinity" for a network that is no longer reachable.
routing loops
Routing Loops
  • Split Horizon:
  • Routing information cannot be sent back in the directionfrom which it was received.

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Split Horizon Rules (1/5)
  • The split horizon rule says that a router should not advertise a networkthrough the interfacefrom which the update came.
routing loops1
Routing Loops
  • Route poisoning:
  • Advertising the downed network as unreachable.
  • When one router receives a route poisoning from another, it sends an update, called a poison reverse, back to the other router.
  • This ensures that all routes on the segment have received the poisoned route information

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route poisoning 1 4
Route Poisoning (1/4)
  • Route poisoning is yet another method employed by distance vector routing protocols to prevent routing loops.
  • Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers.
  • Unreachable is interpreted as a metric that is set to the maximum.
    • For RIP, a poisoned route has a metric of 16.
split horizon with poison reverse 1 5
Split Horizon with Poison reverse (1/5)
  • Now we can put Split Horizon together with Route Poisoning / Poison Reverse.
  • The concept of split horizon with poison reverse is that explicitly telling a router to ignore a route is better than not telling it about the route in the first place.
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Split Horizon with Poison reverse (2/5)
  • The following process occurs:
  • Network becomes unavailable due to a link failure.
  • R3poisons the metric with a value of 16 and then sends out a triggered updatestating that is unavailable.
  • R2processes that update, invalidates the routing entry in its routing table, and immediately sends a poison reverse back toR3.
ways to stop router loops
Holddowns: Prevents regular update messages from reinstating a route that is going up and down (called flapping). Typically, this happens on a serial link that’s losing connectivity and then coming back up.
  • Holddown timers introduce a certain amount of skepticism to reduce the acceptance of bad routing information.
  • If the distance to a destinationincreases (for example, the hop count increases from 2 to 4), the router sets a holddown timer for that route.
  • Until the timer expires, the router will not accept any new updates for the route.
  • This is only one type of timer used with RIP – see next 3 slides:
Ways to Stop Router Loops
rip timers 1 3
RIP Timers(1/3)
  • In addition to the update timer, the IOS implements three additional timers for RIP:
  • Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16.
    • The route is retained in the routing table until the “flush timer” expires.
  • Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.
rip timers 2 3
RIP Timers(2/3)
  • Holddown Timer: This timer stabilizesrouting information and helps prevent routing loops during periods when the topology is converging on new information.
    • Once a route is marked as unreachable, it must stay in holddownlong enough for allrouters in the topology to learn about the unreachable network.
    • By default, the holddown timer is set for 180 seconds.
rip overview
RIP Overview





  • Hop count metric selects the path, 16 is unreachable
  • Full route table broadcast every 30 seconds
  • Load balance maximum of 6 equal cost paths (default = 4)
  • RIPv2 supports VLSM and Discontiguous networks
rip routing configuration
router RIP

router RIP





RIP Routing Configuration

Router(config)#router rip

Router(config-router)#network network-number*

*Network is a classful network address.

Every device on network uses the same subnet mask

rip version 2
RIP Version 2
  • Allows the use of variable length subnet masks (VLSM) by sending subnet mask information with each route update
  • Distance Vector – same AD, and timers.
  • Easy configuration, just add the command “version 2” under the router rip configuration

router rip


version 2

interior gateway routing protocol igrp
Interior Gateway Routing Protocol (IGRP)

Maximum hop count: 255 for larger network, default 100

Composite metric: bandwidth and delay of the line.

Those are the defaults

Also: Load and Reliability are optionally configurable instead

MTU (Maximum Transmission Unit) is a “tiebreaker”

Config t

router igrp 10

discontiguous addressing
Discontiguous Addressing
  • Two networks of the same classful networks are separated by a different network address

  • RIPv1 and IGRP do not advertise subnet masks, and therefore cannot support discontiguous subnets.
  • OSPF, EIGRP, and RIPv2 can advertise subnet masks, and therefore can support discontiguous subnets.
passive interface
Passive Interface

Maybe you don’t want to send RIP updates out your router interface connected to the Internet. Use the passive-interface command:

Router(config)#router rip

Router(config-router)#passive-interface serial0






This allows a router to receive route updateson an interface,

but not send updates via that interface

verifying rip
Verifying RIP

Router#show ip protocols

Router#show ip route

Router#debug ip rip

Router#undebug all (un all)

  • Open your books and go through all the written labs and the review questions.
  • Review the answers in class.